October 15, 2013

Understanding The Link Between Age And Decreased Maximum Heart Rate

As we age, our maximum heart rate (maxHR) decreases, limiting the performance of aging athletes. Decreased maxHR is also a leading cause for nursing home admittance for otherwise-healthy elderly individuals who no longer have the physical capacity required for independent living.

Until now, however, the reasons for decreased maxHR due to age have been unclear. We say that as we age we just slow down, but a new study from the University of Colorado has provided new insight into what exactly is slowing down.

It is well-known that aerobic capacity decreases with age. Think of the chart in most gyms showing the target heart rate for exercising; as you age, that number goes down. That is not a senior discount, it is simply because older hearts can't beat as fast as younger hearts. At 120 beats per minute, an older person's heart is probably working harder — at a higher percentage of maximum heart rate — than the younger person's heart which is at 150 beats per minute.

Catherine Proenza, PhD, and Roger Bannister, PhD, both from the University of Colorado School of Medicine, led the research group. Their findings, published in Proceedings of the National Academy of Sciences (PNAS), reports that one of the reasons for the age-dependent reduction in maxHR is that aging depresses the spontaneous electrical activity of the heart's natural pacemaker, the sinoatrial node.

The study described a dissertation from Eric D. Larsen, a graduate from Proenza's lab in the Department of Physiology and Biophysics.

Larson said, "I utilized a method to record ECGs from conscious mice and found that maximum heart rate was slower in older mice, just as it is in older people. This result wasn't unexpected. But what was completely new was that the slower maxHR was because the individual pacemaker cells — called sinoatrial myocytes, or 'SAMs' — from old mice just couldn't beat as fast as SAMs from young mice."

Tiny electrical signals from the isolated cells were recorded by the research team. They found that SAMs from old mice beat more slowly, even when they were fully stimulated by the fight-or-flight response, which could be observed in these individual cells. A limited set of changes in the action potential waveform -- the electrical signal that is generated by the cells -- was the cause of the slower beating. These change in the action potential waveform were caused by altered behavior of some ion channels in the membranes of the older cells. Ion channels are proteins that conduct electricity across the cell membrane -- much like tiny pinholes in a balloon that open and close to let the air in and out.

As with many initial discoveries in basic science, the results open many more questions and avenues for further research. The important implication of the study is that it raises the possibility that sinoatrial ion channels and the signaling molecules that regulate them could be novel targets for drugs to slow the loss of aerobic capacity with age.

In the interim, Proenza notes that "although maximum heart rate goes down for everybody equally, regardless of physical conditioning, people can improve and maintain their aerobic capacity at all ages by exercising."